1. Field of the Invention
This invention relates to an X-ray mask which is used in an X-ray exposure apparatus or the like.
2. Description of the Prior Art
A conventional X-ray mask used in an X-ray exposure apparatus which is mainly utilized in semiconductor-device production is produced according to the following process.
First, a portion of a mask substrate corresponding to a portion of a membrane formed on the upper surface of the mask substrate, on which a mask pattern is to be formed, is removed by back-etching. Thereafter, an X-ray absorber is selectively provided on the portion where the mask pattern is to be formed on the upper surface of the membrane. After drawing the mask pattern on the X-ray absorber by scanning it with an electron beam, a frame is bonded to the mask substrate.
It has been known that back-etching included in the above-described process produces a warp in the mask substrate due to the tensile stress of the membrane. The amount of the warp is 3 .mu.m when the thickness of the mask substrate is 2 mm, which is a typical value, and becomes at least 6 .mu.m when the thickness is 1 mm, which is thinner than the typical value.
Conventionally, an X-ray mask in which such a warp or distortion is present is used with an apparatus which alters or removes the warp or distortion in the mask during exposure.
FIGS. 16(a) through 16(f) are diagrams sequentially illustrating a conventional X-ray mask production process.
In FIG. 16(a), mask substrate 801 is subjected to backetching after forming membrane 802 on the upper surface thereof. Thereafter, a film of X-ray absorber 803a for forming an X-ray absorber pattern is formed on the upper surface of membrane 802, and resist 812 is coated on X-ray absorber 803a. The X-ray absorber film 803a is formed by etching or plating. A description will now be provided of etching which is generally used.
As shown in FIG. 16(b), mask substrate 801 having the above-described configuration is mounted in cassette 804 having an opening in a portion of the upper surface thereof. Cassette 804 is used for fixing mask substrate 801 when a pattern is formed on X-ray absorber 803a by electron beam scanning. Mask substrate 801 is fixed by pressing its upper surface against the inner surface of the upper plate of cassette 804 by a plurality of leaf springs, such as 805.sub.1 and 805.sub.2, accommodated within cassette 804. Mask substrate 801 maintains the above-described warped state while being accommodated within cassette 804. Electron beam scanning is performed in that state, and a predetermined pattern is formed on X-ray absorber 803a. X-ray absorbing pattern 803b shown in FIG. 16(c) is formed by removing unnecessary portions of X-ray absorber 803a by etching.
As shown in FIG. 16(d), mask substrate 801 on which the above-described X-ray absorbing pattern 803b has been formed is bonded to frame 807. At that time, mask substrate 801 and frame 807 are bonded together using an adhesive, anodic connection or the like and are sandwiched between mask chuck 806 and frame chuck 808 to correct the warp.
Mask substrate 801 and frame 807, bonded together as one body, are then released from mask chuck 806 and frame chuck 808, respectively. At that time, both mask substrate 801 and frame 807 are in a warped state due to the stress of membrane 802, as shown in FIG. 16(e).
Then, mask substrate 801 and frame 807 are mounted on an exposure apparatus, and are utilized for actual exposure. As shown in FIG. 16(f), mask substrate 801 and frame 807 are mounted by suction or magnetic attraction of the lower surface of frame 807 to produce a state in which the warp is corrected. As a result, a patterned region 810, which is irradiated by an electron beam, is deformed in the direction of the outer circumference of the substrate by an amount of pattern shift region 811, causing distortion.
FIGS. 17(a) through 17(f) are diagrams sequentially illustrating another conventional X-ray mask production process.
In the conventional process shown in FIGS. 16(a) through 16(f), an electron beam is used to form a pattern before the mask substrate and the frame are bonded together. In the process shown in FIGS. 17(a) through 17 (f), however, the mask substrate and the frame are bonded together before an electron beam is used to form a pattern.
Since the configuration shown in FIGS. 17(a) through 17(f) is the same as the configuration shown in FIGS. 16(a) through 16(f), the same components as those shown in FIGS. 16(a) through 16(f) are indicated by the same reference numerals, and an explanation thereof will be omitted.
Mask substrate 801 having membrane 802 and X-ray absorber film 803a on the upper surface thereof, shown in FIG. 17(a), is bonded to frame 807 while being sandwiched between mask chuck 806 and frame chuck 808, as shown in FIG. 17(b).
After being bonded together, mask substrate 801 and frame 807 are released from mask chuck 806 and frame chuck 808, respectively. At that time, both mask substrate 801 and frame 807 are in a warped state, as shown in FIG. 17(c). However, the amount of the warp is reduced compared with the configuration shown in FIG. 16(e) because some of the tensile stress of the membrane is absorbed by the rigidity of frame 807.
Then, mask substrate 801 and frame 807 are fixed within cassette 804, as shown in FIG. 17(d), using leaf springs accommodated within cassette 804 as in the case shown in FIG. 16(b). In the present case, however, since frame 807 is accommodated within cassette 804 together with mask substrate 801, leaf springs 905.sub.1 and 905.sub.2 which are shorter than the leaf springs shown in FIG. 16(b) are used. Electron beam scanning is performed to form a pattern on X-ray absorber folm 803a, and excess portions of film 803a are removed to form pattern 803b, as shown in FIG. 17(e).
Mask substrate 801 and frame 807 are then used for actual exposure, but pattern shift region 811, as shown in FIG. 17(f), causes distortion.